In the present state of the air-rectification art, hot air for regenerating carbon-dioxide adsorbers is passed through a bundle of parallel tubes mounted in a housing and in which are inserted heating coils each separately connected to a voltage source. In the event of a break in one of the coils, the entire bundle of tubes must be removed from the housing, thereby completely interrupting operations until the defect has been repaired. A further disadvantage is that the heating coils exert a large drag force on the moving fluid, greatly increasing the pressure gradient along the length of the tubes.
Reference may be had to German printed application (Auslegeschrift) DT-AS No. 16 15 278 and the references therein cited (U.S. Pat. No. 3,270,182 and German utility model--Gebrauchsmuster-DT GM No. 19 54 157) which discloses an electric furnace or gas heater consisting of a bundle of mutually contacting tubes, preferably disclosed in a hexagonal array (hexagonal-close-packed relation) and provided internally with the heating coils.
Such tube bundles can be disposed between the inlet and outlet ends of a housing surrounding the tube bundle and confining the gas to flow through the interiors of the tubes.
Among the disadvantages of such systems are the difficulties in replacing a coil which fails, the need to disassemble the tube bundle in a complicated way and with long downtime of the apparatus for replacement of the tube bundle or one or more tubes or coils thereof, and the considerable resistance to flow of the fluid through the tubes in which the coils function as turbulence-inducing members. The pressure drop between the inlet and outlet sides of the tube bundle is thus high.